1. Field of the Invention
One or more aspects of the embodiments discussed herein relates to a rotor of a brushless direct-current (hereinafter, referred to as “BLDC”) motor, and more particularly, to a rotor of an BLDC motor, which can prevent electromagnetic vibration and noise generated between a rotor and an armature from being transferred to a rotary shaft of the rotor during the motor driving to thereby minimize motor noise and can reduce the weight of the rotor to thereby maximize a motor's power-to-weight ratio.
2. Description of the Related Art
Generally, a conventional rotor of a brushless direct-current (BLDC) motor uses a permanent magnet and a rotor core is necessarily combined with a rotor shaft using a ferromagnetic body or an electric steel sheet in order to form a magnetic circuit of the permanent magnet.
However, when the permanent-magnet rotor generates a rotation torque due to its interaction with a rotating magnetic field of an armature, electromagnetic cogging generated in an air gap between the rotor and the armature, torque ripple, or vibration caused by the interaction of electromagnetism is directly transferred to the rotor shaft and may be then transferred to the load side or may be amplified, thereby causing severe mechanical noise such as resonance noise.
An explanation of the rotor structured as described above will be in detail given. FIGS. 1A and 1B illustrate the structure of a rotor of a conventional BLDC motor, and FIG. 2 illustrates a magnetic circuit of a rotor and an armature 5 of a conventional BLDC motor.
As illustrated in FIGS. 1A and 1B, the rotor of the conventional BLDC motor has a structure in which a radially magnetized C-type permanent magnet 10 is attached to the outer circumferential face of an electric steel sheet ferromagnetic core portion 12 made of a ferromagnetic iron core or armature and a rotor shaft 14 is inserted into a central portion of the ferromagnetic core portion 12.
The C-type permanent magnet 10 is an anisotropic magnet that is magnetized radially around the center of the rotor shaft 14. In order to form a magnetic circuit 102 with a different pole of the rotor, the C-type permanent magnet 10 has to have a ferromagnetic material such as pure iron or an electric steel sheet core provided on its inner circumferential face in FIG. 2.
When the rotor in which the ferromagnetic core portion 12 and the C-type permanent magnet 10 are combined with each other is assembled onto the center of the armature 5, the magnetic circuit 102 through which flux flows is formed as illustrated in FIG. 2. When the pole shift of the armature 5 occurs in magnetic coupling of the formed magnetic circuit 102, the rotor rotates due to interaction torque of a rotating magnetic field.
At this time, vibration caused by unbalance among magnetic flux densities of an air gap 105, a slot portion 15 of the armature 5, and a gap portion 16 of the permanent magnetic 10 of the rotor and magnetizing vibration caused by pole shift of the armature 5 are transferred to the ferromagnetic core portion 12 and a rotor shaft 14 through the permanent magnetic 10. Such vibration is directly transferred up to a load side through the rotor shaft 14, thereby amplifying mechanical vibration noise or causing resonance noise during the motor driving and increasing stress in a bearing while aggravating bearing noise, thus reducing the expected life span of a motor.
In order to reduce vibration noise of the rotor of the conventional BLDC motor, a sound-absorbing resin portion 13 such as rubber or silicon resin is inserted between the ferromagnetic core portion 12 and the rotor shaft 14, thereby blocking noise and vibration transferred through the permanent magnetic 10 and the ferromagnetic core portion 12, as illustrated in FIG. 1B.
In this case, however, the use of the ferromagnetic core portion 12 having a specific area is inevitable in order to minimize resistance between the armature 5 and the magnetic circuit 102 of the C-type permanent magnet 10, as illustrated in FIG. 2.
Moreover, the use of the magnetic core portion 12 cannot greatly reduce a weight of the rotor and the magnetic core portion 12 still acts as a medium through which cogging torque ripple, noise, or vibration generated in the rotor is transferred. As a result, the use of the sound-absorbing resin portion 13 around the rotor shaft 14 for blocking vibration has a limitation in blocking noise and vibration.
Furthermore, for the conventional permanent magnet rotor, in order to combine the C-type permanent magnet 10 with the ferromagnetic core portion 12, a high-strength adhesive has to be used and the weight balance of the rotor may be broken during adhesion between at least two pieces divided from the C-type permanence magnet 10.
The conventional BLDC motor has to use an electric steel sheet core so as to maintain the maximum magnetic flux density of the permanent magnet rotor and to minimize a loss of a rotating electric field. As a result, cogging torque vibration due to interaction with an armature core and electromagnetic vibration noise of the rotating magnetic field are unavoidably transferred to a load side through the motor rotary shaft.